The present invention relates to an image forming apparatus of an electrostatic recording system, an electrophotographic system, etc., and a developing agent used in such an image forming apparatus.
In recent years, the wax content of the toner is increased in order to cope with the high speed operation of a copying machine and a printer and with the oil-free system of a fixing device. If the wax content is increased, the free wax is increased within the developing agent so as to impair the frictional charging between the toner and the carrier. As a result, the charging is shifted toward the zero charging so as to cause the carrier to be attached to the photoreceptor. Also, if the developing agent is stored over a long period of time, the coating agent of the carrier is peeled off, and the wax is attached to the peeled portion so as to impair the charging properties. As a result, the carrier is deteriorated so as to deteriorate the image quality. As a measure for overcoming these problems, it was customary in the past to mix an inorganic oxide with the toner particles so as to allow the inorganic oxide to be attached to the surface of the toner particle. As such an inorganic oxide, it was customary to use, for example, silica particles because silica particles permit the developing agent to be excellent in the charging properties and in the stability during storage of the developing agent over a long period of time. In addition, silica particles are low in the manufacturing cost.
Where silica particles alone are mixed, it is certainly possible to improve the charging properties and the toner fluidity. However, where the addition amount of the silica particles is large and where the adhesion strength of the silica particles to the toner particles is low, the toner attached to the photoreceptor is interlocked, and the interlocked toner causes a black dot to appear on the reproduced image. The surface roughness Rz of the photoreceptor is a large factor for bringing about the particular phenomenon. In other words, the occurrence of the black dot can be markedly suppressed when it comes to a photoreceptor having a small surface roughness Rz.
Also, in order to lower the frictional resistance on the surface of the photoreceptor, it is highly effective to add a metal soap. The addition of the metal soap also permits lowering the frictional resistance with a cleaning blade and, thus, is effective for prolonging the life of the photoreceptor. However, the addition of the metal soap causes contamination of the carrier, making it necessary to decrease the addition amount of the metal soap as much as possible.